Scientists probe quantum sounds

Physicists at the University of Sydney have uncovered a way to use microwaves to probe the sounds of a ‘quantum dot’, the latest promising discovery in the race to build super high performance computers of the future.

A quantum dot consists of a small number of electrons trapped in zero dimensions inside a solid. The quantum mechanical properties of these electrons can be used to store and manipulate data for revolutionary applications in computing, communication, sensing, and bio-medical diagnostic fields, the university said.

James Colless and Xanthe Croot, PhD candidates at the University of Sydney’s School of Physics, are studying what happens when electrons in quantum dots interact with sound waves of the solid material they are trapped in.

Read: The future of computing Read: Scientists demonstrate key component of quantum machine Read: Quantum computers may be commercially available in 20 years: scientist

Professor David Reilly, who is also from the University of Sydney’s School of Physics, said the work is a step further towards understanding the issues that enable and disable quantum machines.

“Sound waves in solids are a key mechanism that can lead to quantum devices interacting with the environment,” he said. “These sound waves are called ‘phonons’, and are similar to the waves one can make in a stretched slinky. The ‘slinky chain’ in this case, is formed by the atoms which make up the solid.

“It turns out that interactions between sound waves and electrons reveal information about the environment of that electron – akin to detecting the size and shape of a room by listening to a singer’s voice in that room,” he said.

However, the interaction between quantum dots and the solids in which they form is a ‘double-edged sword’ for the purpose of quantum computing, according to the researchers.

“On one hand, sound vibrations have been used to shuttle electrons from place to place in quantum circuits, almost like a wave might pick up a surfer and take them into the beach,” said researcher Xanthe Croot.

“However, there are other contexts where sound interacting with electrons can cause huge problems: in particular, when you are performing a quantum algorithm and only want the electron to interact with certain parameters that the experimenter controls,” Croot said.

Unwanted sound can significantly limit the time you have to perform the algorithm before the electron loses all the information it was storing. Understanding how the size and geometry of the quantum circuit affects these interactions is therefore extremely important, Croot said.

In quantum computing, different configurations of electrons within the dot represent something similar to the 0 and 1 (or on and off) states in classical computing. The 1 and the 0 states have different energies: if you apply microwaves with exactly this energy difference you can change the state from 0 to 1 and vice versa.

“We found that if you apply microwaves with energy slightly higher than the electron energy difference, the system creates sound of a very specific frequency. It is almost like the electron saying, if you hit me too hard I’ll scream,” she said

“Changing the microwave energy will change the frequency of the sound that the system creates in the solid. The results show that some frequencies of sound interact very strongly with the system, while others less so. There are hints in the data that the geometry of the quantum dot plays a key role in determining which frequencies will interact strongly.”

The latest findings were published today in science journal, Nature Communications.